Melt butter in a large saucepan over medium heat. Add chopped onion, shredded carrot, garlic, and shallots to pan, and cook for 5 minutes or until vegetables are tender, stirring frequently. Add sugar, pepper, salt, and 4 basil leaves, and cook 5 minutes. Add sun-dried tomatoes, diced tomatoes, and broth, and bring to a boil. Reduce heat, and simmer 1 hour. Remove from heat. Place half of soup in blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Pour into a large bowl. Repeat procedure with remaining soup. Divide soup evenly among 6 bowls. Garnish each serving with 1 basil leaf.